Aging of the male beyond adolescence is accompanied by a progressive, gradual decline in fertility. However, individual variations are pronounced, with many males remaining sexually active into old age, while others may show a decline in fertility at an early age. Although the incidence of degenerative changes in the testes and accessory sex glands increases, and the balance of gonadotropins and sex hormones changes with increasing age, viable sperm are produced over a long period. This is evidenced by the numerous records of offspring sired by men over 70, 80, and some over 90 years old (bull, 22 yr., mouse, 22 months). Current evidence of a reduced fertilizing ability of spermatozoa, per se, with increasing age of males is indirect. The proposal is designed: (1) to directly compare fertilizing ability of sperm from aged and young Syrian hamsters through competitive, heterospermic insemination of females, and (2) to identify the sperm functional components which are affected by aging of the male, primarily through in vitro fertilization experiments and direct examination of spermatozoa and the processs of spermiogenesis with the electron microscope.